Guided missiles that are capable of automatic tracking of their target are typically equipped with a seeker, which includes a sensor and an error-measuring device that indicates a tracking error (i.e. the co-ordinates of the target in the co-ordinate frame of the seeker, the target being at the origin of that frame when the target is dead ahead of the missile). The seeker receives electromagnetic radiation that is reflected or otherwise emanates from the target, and a guidance law is used to calculate a control signal that must be applied to the missile's steering mechanisms in order for the missile to intercept the target.
There is a need for a new generation of very small anti-surface missiles that can acquire and track military targets such as vehicles and defence installations. Because of size and cost constraints, current missiles of that kind generally either use command guidance (i.e. there is no seeker inside the missile) or a very simple seeker such as a semi-active laser receiver. There is a need for simpler, lower-cost seeker concepts to meet the needs of future lightweight weapons. Passive infrared- or visible-band seekers can provide autonomous operation, so that the operator does not need to keep a line of sight to the target, but they suffer from the disadvantage that their performance is degraded when visibility is poor, e.g. due to fog, rain or battlefield smoke. Radar seekers do not suffer from that disadvantage, but prior-art radar seekers are typically large, costly and provide insufficient resolution to provide reliable acquisition of the correct target.
A seeker typically measures the position of a target by determining the angle of arrival of the electromagnetic radiation arriving at the seeker from the target. The determination of that angle can be achieved in a number of different ways. In conical scanning (conscan) radar, a narrow radar beam is rotated around the radar boresight, typically by mechanical means. The measured amplitude of the signals received at the radar from the target will vary with the angle between the boresight and the line of sight of the target; when the boresight is on the target, the measured amplitude will be constant as the beam rotates. In a monopulse radar, a plurality of (typically four) receiving antennas are arranged adjacent to each other, equally spaced around a central point. A radar pulse echo from a target directly on the boresight falls equally on all of the antennas, whereas a pulse echo from a target angularly displaced from the boresight falls more on some of the antennas than on others. The relative signal amplitudes received from each antenna therefore vary according to the position of the target. A comparator and signal processor is used to derive the angle of arrival of the target signal. A monopulse radar requires a multi-channel radar receiver, which can be large and expensive. Also, it can be difficult and expensive to achieve good accuracy with a monopulse radar: angle measurements are corrupted by thermal noise at low signal-to-noise ratios and high tolerances are required in the gimbal mechanism, the servo pickoffs and gyros, the antenna and the radome aberration, all of which tends to drive up the cost and complexity of the seeker.
Cost savings could be achieved by eliminating the need for a gimbal mechanism to steer the antenna. Phased-array antennas, in which steering is achieved by altering the relative phases of an array of antenna elements, do not require a gimbal mechanism. However, in the field of guided missiles, phased-array antennas suffer from a number of significant disadvantages. In particular, their available aperture is small, which makes it difficult to achieve sufficient precision in steering for reliable target tracking accuracy.
It would be advantageous to provide a missile seeker and guidance method in which one or more of the aforementioned disadvantages is eliminated or at least reduced.